Gunning Fog Index Readability Formula
Short sentences and short words written in plain English are readable by more people than long sentences and long words written in complicated language.
The Gunning Fog Index measures grade-level reading ability. The general average is 7. The Bible , Shakespeare, and Mark Twain have Fog Indexes of
6. The leading magazines, like Time and Newsweek , and The Wall Street
Journal average 11.
Guideline
7 – easy to read by most people
10 – average 15-year-old
11–13 – college student
14–16 – university student over 18 – too difficult for newspapers and magazines
To Count the Fox Index and the Grade Level
In a passage of at least 100 words, count the number of words and the number of sentences.
Count the number of words of three or more syllables that are not proper nouns, are not combinations of easy words or are hyphenated words, and are not two-syllable words made into three- with es and ed endings.
To determine the Average Sentence Length (ASL), divide the number of words by the number of sentences. ASL = W/S
To determine the Percent of Hard Words (PHW), divide the number of hard words by the number of words. PHW = HW/W
To determine the Grade Level (GL).
GL = 0.4 x (ASL + PHW)